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From: Blue Swirl <blauwirbel@gmail.com>
Date: Mon, 17 Oct 2011 21:32:00 -0000
Message-ID: <CAAu8pHv2gG6o0k-MbDxtOPTiL8k9s_RgSp2w0O2ZkQM+RWN1rw@mail.gmail.com>
Subject: Re: [Qemu-devel] gcc auto-omit-frame-pointer vs msvc longjmp
To: Richard Henderson <rth@twiddle.net>
Cc: Bob Breuer <breuerr@mc.net>, Mark Cave-Ayland <mark.cave-ayland@siriusit.co.uk>, 	Kai Tietz <ktietz70@googlemail.com>, qemu-devel <qemu-devel@nongnu.org>, 	"gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
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On Mon, Oct 17, 2011 at 5:22 PM, Richard Henderson <rth@twiddle.net> wrote:
> On 10/17/2011 07:09 AM, Bob Breuer wrote:
>> I don't think this is a free/g_free issue. =C2=A0If I use the following
>> patch, then I at least get the openbios messages:
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index a9fa608..dfbd6ea 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -180,6 +180,7 @@ static void cpu_handle_debug_exception(CPUState
>> =C2=A0/* main execution loop */
>>
>> =C2=A0volatile sig_atomic_t exit_request;
>> +register void *ebp asm("ebp");
>>
>> =C2=A0int cpu_exec(CPUState *env)
>> =C2=A0{
>> @@ -233,6 +234,8 @@ int cpu_exec(CPUState *env)
>>
>> =C2=A0 =C2=A0 =C2=A0/* prepare setjmp context for exception handling */
>> =C2=A0 =C2=A0 =C2=A0for(;;) {
>> + =C2=A0 =C2=A0 =C2=A0 =C2=A0int dummy =3D 0;
>> + =C2=A0 =C2=A0 =C2=A0 =C2=A0ebp =3D &dummy;
>
> See if
>
> =C2=A0asm("" : : : "ebp");
>
> also solves the problem.
>
>> Google finds a mention of longjmp failing with -fomit-frame-pointer:
>> http://lua-users.org/lists/lua-l/2005-02/msg00158.html
>>
>> Looks like gcc 4.6 turns on -fomit-frame-pointer by default.
>
> Hmm. =C2=A0This is the first I've heard of a longjmp implementation
> failing without a frame pointer. =C2=A0Presumably this is with the
> mingw i.e. msvc libc?
>
> This is something that could be worked around in gcc, I suppose.
> We recognize longjmp for some things, we could force the use of
> a frame pointer for msvc targets too.
>
> For now it might be best to simply force -fno-omit-frame-pointer
> for mingw host in the configure script.

IIRC buggy versions of alloca() could also fail without a frame pointer.